1. Field of the Invention
The invention relates to the field of producing shaped products from recycled plastic material, and more particularly to a method and apparatus for extruding shaped products from plastic material, particularly recycled materials including plastic.
2. The Prior Art
Plastic materials are sometime recycled by carefully sorting one type of refuse material from another, and ultimately producing granules of pure plastic of a particular type or grade. This is generally not an economical way to recycle plastics from household garbage since it is too costly to adequately separate and sort the different plastics and other materials which are present in household waste. Some uses are known for recycled impure waste plastics, sometimes mixed with other waste products. Typically this involves melting the plastics and pouring the melted plastics into molds to form products such as paving tiles or picnic tables. The process of pouring a mold and allowing the plastic to cure and cool, and then removing the product from the mold is a lengthy process which adds considerably to the cost of the end product and reduces its economic efficiency.
A method of continuously producing an elongated beam from recycled plastic material is disclosed in U.S. Pat. No. 5,413,745, issued to Curt Andersson, in which a mixture of inexpensive waste plastic and dried and heated waste fiber material such as paper is melted and compressed in a machine, and extruded in the form of a beam. The waste plastic material and the waste fiber material are preferably previously shredded or chopped up prior to feeding the same to the machine. In the Andersson process, a mixture of shredded plastic and filler material is fed by a screw feeder to a compression area where a screw conveyor of decreasing pitch further compresses the material, so that it is heated and melted. The molten mixture is continuously forced out of a square or rectangular nozzle in the form of a beam, after which moveable wall sections of a matrix are used to keep the beam shape of the material until it cools. According to Andersson, the beam of material coming out of the nozzle has a low mechanical strength, and the matrix is designed to work and cool the beam to improve its mechanical strength. The Andersson method is suitable for producing continuous elongated beams but the beams are still lacking the rigidity and strength required for structural purposes.
Canadian Patent No. 2,119,512, issued to Roman Evancic, provides a method and apparatus for producing shaped articles from recycled materials. The method comprises: producing a mixture of plastic and fibrous materials wherein the proportion of plastic to the fibrous materials is greater than approximately 10% and less than 60%; heating the mixture to a softened state adapted for extrusion; extruding the materials from an extrusion device through a shaping means to produce a sheet of softened extruded material; cooling and compressing the surface of the sheet to form a skin of relatively harder material on the surface of the extruded material; applying rollers having a low co-efficient of friction to the surface of the sheet under pressure to apply the desired shape to the surface of the sheet; and further cooling the sheet by the application of a cooling liquid.
Other patents of interest are U.S. Pat. No. 5,786,000, issued to Rolf Berner and U.S. Pat. No. 5,788,901, issued to Barnard et al.